Modulatory neurotransmitters such as dopamine and 5-HT are implicated in serious neuropathologies that range from migraine, depression, schizophrenia, memory impairment, epilepsy, and defects in sensory and motor processes. Receptors, particularly G protein coupled receptors (GPCRs), are the presynaptic and postsynaptic targets of these neurotransmitters. GPCRs are the largest family of cell surface receptors. Therefore, understanding how these receptors exhibit their myriad effects is vital. Defects in signaling involving 5-HT1B receptors, in particular, are of note in all the pathologies above. This receptor is presynaptically localized and inhibits transmitter release. An underlying molecular mechanism of this inhibition has been determined in axons of a primitive vertebrate, the lamprey. In lamprey, 5-HT inhibits transmitter release by a direct action of Gpy on the essential vesicle fusion machinery, the SNARE complex, to cause a change in mode of synaptic vesicle fusion and reduce the evoked cleft glutamate concentration. However, it is unknown whether this mechanism is conserved in mammals. Understanding of the biochemical cascade in the mammalian CNS could provide a model for drug intervention. In hippocampus, the 5-HT1B receptor is expressed on presynaptic terminals of CA1 pyramidal cells that synapse locally and also project to the subiculum and entorhinal cortex. The 5-HT1B receptor reduces the synaptic cleft glutamate concentration without affecting release probability, which suggests a shift in mode of fusion. The classical model of transmitter release predicts vesicle fusion occurs through full fusion, but recent advances indicate an alternative mode, kiss-and-run, may occur in the CNS. I intend to determine the effect of reduced cleft glutamate concentrations on postsynaptic activity and hypothesize that NMDA and AMPA receptor responses are not impacted equally by 5-HT. The inaccessibility of the presynaptic terminal makes examination of individual synapses difficult. To overcome this limitation, I will analyze Ca2+ transients in single dendritic spines. Furthermore, the presynaptic mechanism most commonly attributed to GPCR mediated inhibition is presynaptic attenuation of Ca2+ entry. Thus, I will similarly investigate presynaptic Ca2+ entry to determine processes that mediate the 5-HT-mediated reduction in neurotransmitter release. Finally, to understand the molecular mechanism by which 5-HT1B receptor reduces cleft glutamate concentration, we will probe the vesicle fusion machinery using botulinum toxin A, which cleaves a part of the protein that makes the SNARE complex, where G|ty is known to interact with and finally a carrier technique will be used to place peptide inhibitors to interfere with Gpy-SNARE complex interactions.